1. Field of the Invention
The present invention relates to an organic light emitting diode (OLED) for realizing a full-color display device, and more particularly, to a polymer OLED and a method for producing the same.
2. Description of the Related Art
OLEDs which can realize full-color display devices are largely divided into two types: OLEDs employing low molecular weight materials; and OLEDs employing polymer high molecular weight materials.
A high molecular weight OLED is generally fabricated such that two opposite electrodes, a cathode and an anode, are disposed on a substrate. A hole transport layer (HTL) and an emission layer are provided between the anode and the cathode. In the polymer high molecular weight OLED, the HTL and the emission layer are formed of organic polymers. Recently, research into polymer OLEDs has been actively carried out because they drive at a relatively lower voltage, consume a relatively small amount of power and can easily realize large, full-color display screens.
Organic layers, both as active and passive matrix types on the basis of polymer OLEDs, are fabricated according to the state of the art with printing techniques such as ink-jet printing.
In this known method, the light emitting polymers are liquefied to a so-called polymer ink. The polymer ink is printed onto a substrate via an ink-jet printing head.
The OLED, in the simplest case of a passive matrix display screen, is fabricated as follows.
First, a transparent substrate made from glass or plastic is coated with a transparent conductor material such as indium tin oxide (ITO) to form an anode having a predetermined pattern.
In a next step, a hole transport layer (HTL) is formed of an organic material, for example, poly-(2,4)-ethylene-dihydroxy thiophene (PEDOT) or polyaniline (PANI) on the anode. The HTL is applied onto the anode on the substrate by means of ink-jet printing or spin coating.
A polymer emission layer is then formed on the organic HTL by the ink-jet printing method as mentioned above. In order to obtain a full-color display screen, a red-emitting, a green-emitting and a blue-emitting polymer is imprinted on the organic HTL. Following this, a cathode, formed from a layer of calcium and a layer of aluminium, is vapour-deposited over the polymer emission layer.
Finally, the entire structural element is encapsulated. The cathode and the anode are then connected to a driving electronic system.
In order to print the polymers for each pixel in a predetermined lattice or line array, a structure is formed of an organic or inorganic substance, such as a photo-resist as shown in FIGS. 1A and 1B. The photo-resist structure forms the peripheral zone of the pixels for pixels that are lattice-patterned. If the pixels are line-patterned, the photo-resist structure forms the left and right boundary limitations of the lines.
FIGS. 1A and 1B shows a portion of the structure of an OLED without organic polymers layers. A first electrode layer 2 having a predetermined pattern is formed on a glass substrate 1. A first insulator layer 3 made from the photo-resist, and a second insulator layer 4 forming channels 40, are formed on the first electrode layer 2. Predetermined openings 31 are formed in the first insulator layer 3 by exposing and developing steps so that a predetermined area of the first electrode layer 2 is exposed through the openings 31 to define pixels.
Referring to FIGS. 2A and 2B, polymer material layers, that is, a hole transport layer 5 and a polymer emission layer 6, are formed in the channels 40.
So-called multi-channel printing heads are used to obtain effective printing. With these printing heads, it is possible to simultaneously print a plurality of pixels. Accordingly, several jets of the printing head are provided. With a piezo-actuated printing head, the piezo actuators of various jets are activated with subsequent excitation of the drop formation from these various jets. For printing, the substrate is moved with constant speed opposite the printing head. In this case, the printing head prints into the lines or lattices of the pixels on the substrate in accordance with the number of active jets of the printing head. As shown in FIGS. 2A and 2B, the lines or lattices are printed into the channels pre-structured by the second insulator layer 4. The second insulator layer 4 ensures that the polymer ink does not flow into neighbouring channels. In this way, red, green and blue emitting polymers can be printed in a line-shaped manner next to each other without causing any mixing of colours.
The fabrication of organic light emitting diodes on a polymer basis is described in the patents EP 0423283 and WO 9013148. The fabrication of OLEDs by means of printing methods, such as ink-jet printing, is described in the patents EP 0908725, EP 0940796, EP 0940797, EP 0989778, WO 9943031, WO 9966483, WO 9828946, U.S. Pat. No. 6,087,196, WO 0012226, and WO 0019776.
The fabrication of photo-resist structures is described in the EP 0996314 A1.
By the boundary limitation of the various rows or columns of a full-color display screen using an insulator layer, e.g., photo-resist, it is ensured that HTL ink as well as polymer ink can be printed into the pre-structured channels. In this way, red, green and blue emitting polymers can be printed in a lattice or line array next to each other without polymer flowing into the neighbouring channels or causing a mixing of colors in the process. In other words, it is ensured that no ink can flow over this photo-resist barrier and into the neighbouring lines. The limiting photo-resist structures form the channels, and form lattice or line arrays on the substrate that is then built up to the full-colour display screen.
As shown in FIGS. 2A and 2B, the channels are open at the upper and lower rims. The photo-resist structures only provide lateral border limitations for the channels. The HTL ink and the polymer ink can easily run out from the upper and lower rims of the opened channels. Therefore, the amount of ink at the upper and lower rims of the channels is less than that at central portions of the channels. Accordingly, after the HTL ink and the polymer ink are dried, the HTL and the emission layer exhibit nonuniformity in layer thickness at the upper and lower rims thereof, which are effective areas for electro-luminescence emission. This occurs because the layer thicknesses of the HTL- and polymer emission layer gradually become thinner due to the HTL ink and polymer ink running out from the ends or rims of the channels.